Every displacement pump, when opening the chambers, pulsates into the outlet (pressure side of the pump), regardless of the geometric pulsation. For this, the following conditions must be given:—the media to be conveyed is compressible;—there is a pressure difference between the outlet and the opening chamber of the pump. When opening the chamber into the outlet, there is a pressure equalization leading to a pulsation. The pressure may be higher in the last chamber upstream of the outlet than in the outlet. When opening the chamber, a pressure impulse then occurs in the direction towards the outlet of the conveying mechanism. On the other hand, the counter pressure in the outlet can be higher than in the last chamber, so that a pressure impulse occurs into the opening chamber. These recurring pressure impulses are referred to as pulsation. They ultimately affect the accuracy of the discharged portions because a different number of pressure impulses occurred within the discharged separated portions.
The greater the pressure difference, the chamber size and the compressibility of the medium, the greater the pulsation in the volume flow. If such a pump is used for producing individual portions, this inevitably leads to weight differences of the individual portions.
In order to prevent pulsation in the event of increased counter pressure in the outlet, the chamber pressure or the compression, respectively, can be increased and thus be adapted to the outlet pressure. When opening the chamber, pressure equalization in the outlet thus practically no longer occurs.
In conventional vane cell pumps, as is particularly evident from FIG. 5, increased pressure is generated in the chambers which convey the pasty substance. The inlet for the pasty substance into the conveying mechanism is followed by a compression zone. By compressing the substance to be filled, i.e. by reducing the volume of the closed chambers, the pressure in the chamber is increased. In this, the radius of the outer contour, along which the pump vanes brush and seal the chamber, is reduced.
It is assumed in a simplified manner, that the compressibility of the medium to be filled is caused by its gas portion. This gas portion is usually air which is incorporated (intentionally or unintentionally) during production of the medium, e.g. sausage meat. For calculating the chamber pressure, it can therefore be assumed in a simplified manner for the gas portion of the medium, that: p×v=const.
It has already been attempted to change the compression. A groove or pocket is disposed in the top side plate, i.e. in the cover of the vane cell pump, on the side facing the chambers, so that a connection between the individual vane cell chambers and the inlet region is given. It can be influenced by means of the length of this pocket, that the chamber is closed later or earlier, or that the compression is lower or higher, respectively. However, since a conveying mechanism always conveys different media with varying compressibility, a pressure relief valve (see FIG. 5) must be installed via which, in case of overcompression, excess substance can be returned through the return channel into the supply region. This leads to unwanted stress and damage of the medium to be filled and can, for example, for raw sausage lead to “smudgy end products”. In addition, this pocket in the side plate reshapes substance (depending on the length of the pocket and the compressibility of the medium), which can additionally be damaging, especially for sensitive products.
For changing the compression, a rotor with one or more or vanes or cells, respectively, can be used. In this manner, the chambers generally become smaller or larger. For the same reduction of the closed chamber, the chamber pressure increases all the more, the smaller the chamber was before the compression—and vice versa:V2=V1−V compression,P2=V1×P1/(V1−V compression)
where P1, V1 are pressure and volume prior to compression,
P2, V2 are pressure and volume after compression, and
V compression is the compressed volume.
This, however, results in the following drawback: In order to have no geometric pulsation of the conveying mechanism, the pump interior contour can be geometrically configured to only one specific number of vanes. This means that weight fluctuations occur when a rotor having more or fewer vanes is used. In addition, a corresponding conversion is costly.